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The Basics of Surface Mount Assembly

27th February 2014

Using surface mount assembly – also known as surface mount technology or SMT – for the production of printed circuits began in the 1960s. Much of the pioneering work on this method of printed circuit board assembly was carried out at IBM. Components were designed with metal tabs that could be soldered directly to the PCB. This made for easier and faster assembly than was possible with the earlier through-hole production, where components had connectors that passed through the board and were soldered from the rear. This carried the additional advantages that circuits could be made smaller and could be double-sided.
The printed circuit boards used for SMT assembly have flat metal pads where the components are to be placed. A solder paste – a mix of solder and flux – is applied to these using a stencil or via a jet-printing process. The components are then put in place, held temporarily by the stickiness of the paste. The next stage is a soldering oven, which heats the solder paste and fixes the components in place. If surface mount assembly is used to create a double-sided board, this process may be repeated. Once soldering is complete, the board is generally given a wash to remove any surplus flux and solder that may cause short circuits.
Using surface mount assembly has a number of advantages. Component densities can be higher, and the components themselves can be smaller. It also means that production can be more automated and thus faster. Some machines are capable of placing more than 100,000 components an hour. This means SMT assembly is ideal for high-volume production runs. Being a largely automated process, it produces fewer errors too, leading to higher reliability for the finished circuits.
There are some drawbacks too. Prototype assembly is more difficult, for example, and requires skilled operators and specialist tools. Repair of boards is harder too because of the small size of components. And surface mount assembly isn't suited to high-voltage parts such as those used in power circuitry. It's also not suitable for attaching components that are subject to stress, such as connectors used to attach other devices.
To learn more about how we can help you create boards using surface mount assembly and about our other services, take a look around the website.